A conventional motor driving apparatus drives a motor by using a common system comprising a single-phase rectifier circuit and an inverter. The single-phase rectifier circuit has a reactor for improving a power factor (not shown), and a smoothing capacitor for smoothing an output voltage of the single-phase rectifier circuit.
In the conventional motor driving apparatus so constructed, the smoothing capacitor and the power-factor improvement reactor have large capacitances, resulting in problems in the cost, lifetime, efficiency, weight, size, and the like of the motor driving apparatus. The large-capacitance smoothing capacitor and the large-capacitance reactor are employed to prevent degradation in the power factor due to distortion in the waveform of the current that flows from the power supply to the motor driving apparatus. In other words, in the motor driving apparatus, when the values of the smoothing capacitor and the reactor become small, the power factor is degraded due to distortion in the waveform of the current that flows from the power supply to the motor driving apparatus, leading to an increase in harmonic components. As a result, the motor driving apparatus cannot satisfy the IEC (International Electrotechnical Commission) harmonics standards.
Thus, there has been proposed a method for drastically decreasing the capacitances of the smoothing capacitor and the power-factor improvement reactor, and further, for improving the input power factor of the motor driving apparatus (for example, Japanese Published Patent Application No. 2002-51589).
Hereinafter, a description will be given of a motor driving apparatus using the method disclosed in Japanese Published Patent Application No. 2002-51589 (first prior art).
FIG. 11 is a block diagram for explaining a motor driving apparatus according to the first prior art.
With reference to FIG. 11, a motor driving apparatus 100 includes a single-phase rectifier circuit 3 having an input connected to a single-phase AC voltage (power) supply 1, and an inverter circuit 4 that is connected to the single-phase rectifier circuit 3 and outputs a current and a voltage to a motor 2.
The single-phase rectifier circuit 3 includes first and second diodes 31 and 32 connected in series, and third and fourth diodes 33 and 34 connected in series. The cathodes of the first and third diodes 31 and 33 are connected to each other, and the connection node is an output node 3a of the single-phase rectifier circuit 3. The anodes of the second and fourth diodes 32 and 34 are connected to each other, and the connection node is the other output node 3b of the single-phase rectifier circuit 3. A smoothing capacitor 12a is connected between the output nodes 3a and 3b of the single-phase rectifier circuit 3. Further, an output terminal of the single-phase AC voltage supply 1 is connected to the connection node 3c of the first and second diodes 31 and 32, while the other output terminal of the single-phase AC voltage supply 1 is connected to the connection node 3d of the third and fourth diodes 33 and 34.
Further, the inverter circuit 4 comprises first and second switching elements 41 and 42 connected in series, third and fourth switching elements 43 and 44 connected in series, and fifth and sixth switching elements 45 and 46 connected in series. One end (higher-voltage-side terminals) of each of the first, third, and fifth switching elements 41, 43, and 45 are connected to each other, and the connection node (one input node) is connected to the one output node 3a of the single-phase rectifier circuit 3. One end (lower-voltage-side terminals) of each of the second, fourth, and sixth switching elements 42, 44, and 46 are connected to each other, and the connection node (the other input node) is connected to the other output node 3b of the single-phase rectifier circuit 3. Further, first to sixth diodes 51 to 56 are connected in inverse-parallel to the first to sixth switching elements 41 to 46, respectively. The connection node 4a of the first and second switching elements 41 and 42 is a first output node of the inverter circuit 4, the connection node 4b of the third and fourth switching elements 43 and 44 is a second output node of the inverter circuit 4, and the connection node 4c of the fifth and sixth switching elements 45 and 46 is a third output node of the inverter circuit 4. The first to third output nodes 4a to 4c of the inverter circuit 4 are the input nodes of the respective phases of the three-phase input of the motor 2.
The motor driving apparatus 100 further includes a current command calculation unit 14 for outputting a current command value io on the basis of an absolute value |v| of a voltage outputted from the single-phase AC voltage supply 1, a command torque To supplied from the outside (i.e., externally supplied), and a current (DC link current) idc that flows between the single-phase rectifier circuit 3 and the inverter circuit 4. The motor driving apparatus 100 also includes a current control unit 15 for outputting a drive signal (gate signal) Sg to the gates of the respective switching elements 41 to 46 of the inverter circuit 4 on the basis of the current command value io, and a current i that actually flows in the motor 2.
The current command calculation unit 14 modulates the command torque To supplied from the outside by the absolute value |v| of the output voltage v of the single-phase AC voltage supply 1 to generate a modulated torque waveform, and calculates a current command value io so that the waveform of the DC link current idc becomes equal to the waveform of the modulated torque. The current control unit 15 compares the current command value io calculated by the current command calculation unit 14 with the current i that actually flows in the motor 2, and controls the inverter circuit 4 with the gate signal Sg so as to eliminate a deviation between the current command value io and the current i. Actually, the current control circuit 15 performs control such as a three-phase to two-phase conversion of the current i to be controlled.
In the motor driving apparatus 100 (first prior art), the waveform of the DC link current idc that flows between the single-phase rectifier circuit 3 and the inverter circuit 4 becomes equal to the waveform of the absolute value |v| of the voltage v outputted from the single-phase AC power supply 1, whereby the current waveform of the single-phase AC power supply 1 is improved, resulting in an increase in the power factor. Therefore, the capacitances of the smoothing capacitor and the power-factor improvement reactor can be reduced.
However, when the capacitance of the smoothing capacitor is reduced, the input voltage of the inverter circuit 4 pulses. As a result, the level of the input voltage of the inverter circuit 4 is lowered, and a desired voltage to be applied to, for example, a brushless DC motor, cannot be obtained.
In order to solve this problem, there has been proposed a motor driving apparatus (second prior art) for advancing the phase of the output voltage when the output voltage of the inverter circuit is saturated (for example, Japanese Published Patent Application No. Hei. 10-150795).
In this motor driving apparatus (second prior art), when the output voltage of the inverter circuit that outputs the motor driving voltage is saturated, i.e., when the level of the output voltage of the inverter circuit becomes equal to or higher than the level of the input voltage, the phase of the motor driving voltage (inverter output voltage) is advanced to set the brushless motor in a so-called weak field state, thereby reducing the level of the driving voltage which is required for the brushless motor. Therefore, even when the input voltage of the inverter circuit 4 is small, the output voltage of the inverter circuit 4 is prevented from becoming saturated, whereby the motor can continue to drive.
In the motor driving apparatus 100 as the first prior art, however, a charging current to the smoothing capacitor 12a is not considered. Therefore, satisfactory effects of power factor improvement cannot be obtained by only modulating the motor driving current so that the waveform thereof becomes equal to the waveform of the input power supply voltage by using the absolute value of the inputted power supply voltage.
Further, in the first prior art, since modulation of the motor driving current is realized by matching the waveform of the DC link current idc that flows between the single-phase rectifier circuit 3 and the inverter circuit 4 to the modulated waveform of the command torque To, detection of the DC link current idc is indispensable. Moreover, control of the motor driving current is very complicated, that is, control of the motor driving current comprises the steps of calculating the current command value io on the basis of the DC link current idc and the modulated waveform of the command torque To, and adjusting the gate signal Sg to be applied to the inverter circuit 4 so as to eliminate a deviation between the calculated current command value io and the actual driving current i.
Furthermore, since the motor driving current is vigorously modulated by using the output waveform of the AC power supply, the output torque of the motor is also modulated by the output waveform of the AC current supply, resulting in a possibility that significant noise and vibration might occur in a high-load area. Moreover, since the motor driving current is modulated, the threshold torque that can be output from the motor controlled by the motor driving apparatus might be reduced.
Furthermore, since the command torque To is modulated by the absolute value |v| of the power supply voltage v of the command torque To, the instantaneous value of the power supply voltage must be detected. As a result, an AD converter for detecting an analog value and a microcomputer for detecting the instantaneous value on the basis of the output of the AD converter are required, leading to an increase in cost.
On the other hand, in the motor driving apparatus as the second prior art, when the output voltage of the inverter circuit is saturated and the induced voltage of the motor is increased, the phase of the output voltage is advanced so as to maintain the current supply to the motor. However, this leads to a reduction in efficiency of the motor. That is, even when the output voltage of the inverter circuit becomes lower than the induced voltage of the motor, the motor driving current continues to flow and the torque continues to occur for a while because of a reactance component existing in the motor. At this time, advancing the output voltage of the inverter circuit leads to an increase in the phase difference between the output current and the output voltage, resulting in a reduction in the motor driving efficiency. Moreover, advancing the phase of the output voltage of the inverter circuit to prevent saturation of the output voltage is technically very difficult.
Further, the second prior art has the following fundamental problem. That is, in the state where the regenerative current of the motor flows, there is a possibility that the input voltage of the inverter circuit is increased by charging and thereby the output voltage of the inverter circuit is not saturated. In this case, the output voltage of the inverter circuit is not saturated even in the section where the regenerative current flows and the phase of the voltage to be applied to the motor must be advanced, and therefore, the motor driving apparatus stops the operation of advancing the phase of the output voltage of the inverter circuit. As a result, there is a possibility that the regenerative current cannot be stopped.
Furthermore, in the second prior art, even when the phase of the output voltage of the inverter circuit is advanced, the state where the output voltage of the inverter is lower than the induced voltage of the motor is maintained. When a predetermined period of time has passed under this state, a backward current flows from the motor to the inverter circuit, and thereby the power is regenerated. Since the current that causes this regenerative power applies a brake to the motor, the motor driving efficiency is lowered. Moreover, since, during this regeneration period, no current is supplied from the single-phase AC power supply to the inverter circuit, the current waveform of the single-phase AC power supply is distorted, resulting in a reduction in the input power factor of the inverter circuit.